This invention relates generally to an improved regulating scheme for stabilizing the operation of a current fed induction motor drive system and, more particularly, to apparatus for balancing the motor air-gap flux against motor stator current whereby a stable operating point is achieved regardless of available supply voltage or commanded torque levels.
An induction motor typically comprises a squirrel cage or wound rotor that is mounted in a stator having windings connected to a suitable source of excitation. Excitation of the stator windings creates a magnetic flex across the stator-rotor air-gap of the motor and the current induced in the rotor interacts with the air-gap flux to produce an electromagnetic force or torque tending to move the rotor relative to the stator. The amount of torque developed by the motor is often expressed in terms of the magnitude of the air gap flux and the slip frequency between the stator and rotor. The effective slip frequency by definition is the difference between the frequency of the flux wave on the air gap and the equivalent electrical frequency at which the motor shaft is rotating (i.e., motor speed). Where such a motor is required to run at variable speeds with variable loads and in both forward and reverse directions, as in the case of traction motors for electrically propelled vehicles, the stator windings are advantageously supplied with polyphase a-c power which is so conditioned that the frequency as well as the amplitude of the stator excitation are adjustable as desired and the phase sequence is reversible.
A review of control systems for current fed induction motor drives is provided in U.S. Pat. No. 4,088,934 issued for J. D. D'Atre, T. A. Lipo, and A. B. Plunkett and assigned to General Electric Company. That patent teaches stabilizing a current fed induction motor drive system by controlling the excitation source of the motor as a function of the actual phase angle between the air gap flux and the stator current in the motor. The control system varies the frequency of the current supplied to the stator so as to regulate the phase angle to a desired value. In addition, the magnitude of air-gap flux is directly monitored and the magnitude of excitation applied to the stator is controlled so as to regulate the magnitude of air-gap flux to a desired value. However, the magnitude of air-gap flux, although regulatable by varying the stator current, is affected by the frequency of the stator excitation and the motor slip frequency. Therefore, a feedback control system which merely varies motor excitation in order to regulate air-gap flux does not necessarily cause the motor to be operated on a preferred operating characteristic.